The present invention relates to a new process for the production of mixtures of 2,4- and 2,6-diisocyanato toluene, which are characterized by an especially high content of 2,6-diisocyanato toluene. The present invention is also directed to the use of these mixtures as synthesis components in the production of polyurethane elastomers.
Diisocyanato toluenes or tolylene diisocyanates (abbreviated to "TDI") are commercially and economically extremely important starting materials for the large scale production of polyurethane synthetic materials. They are obtained on a large scale by phosgenation of the corresponding diamino toluenes or tolylene diamines (abbreviated to "TDA"). Until now, "TDI 100," "TDI 80," and "TDI 65" that is, respectively, pure 2,4-diisocyanato toluene, its mixture having 20% of 2,6-diisocyanato toluene, and its mixture having 35% of 2,6-diisocyanato toluene, based on the total mixture have, in particular, played a major commercial role.
TDI having a content of more than 35% by weight of 2,6-diisocyanato toluene is already known. Thus, for example, U.S. Pat. No. 3,022,328 describes the partial reaction of TDI 65 with tertiary alcohols and the isolation by distillation of the tolylene diisocyanate isomer mixture having an increased concentration of 2,6-diisocyanato toluene from the reaction product obtained. According to U.S. Pat. No. 3,554,872, an analogous concentration of the 2,6-isomer results from the partial reaction of TDI with polyester polyols or polyether polyols and by the subsequent distillative working up of the reaction product.
As was found, the production of diisocyanato toluene isomer mixtures having a content of from 36 to 90%, preferably from 45 to 85% by weight of 2,6-diisocyanato toluene from diisocyanato toluene isomer mixtures having a maximum content of 35% by weight of 2,6-diisocyanato toluene can be carried out, in a particularly simple way, by the partial trimerization of the isocyanate groups of the initial mixture and by the subsequent isolation of the unconverted diisocyanate having an increased concentration of 2,6-diisocyanato toluene.
It was also found that the diisocyanato toluene isomer mixtures which can thus be obtained or according to any processes of the prior art, having a 2,6-diisocyanato toluene content particularly of from 45% to 85% by weight using conventional higher molecular polyhydroxyl compounds of polyurethane chemistry and chain lengthening agents can also be processed into high grade polyurethane elastomers, when simple glycols are used as chain lengthening agents instead of conventional diamines. Until now, the production of high grade polyurethane elastomers from TDI, higher molecular polyols and low molecular glycols as chain lengthening agents was not possible. Instead, until now, in order to produce elastomers of this kind, either expensive and often physiologically harmful diamine chain lengthening agents or special diisocyanates (for example, 1,5-diisocyanato naphthalene) had to be used in order to produce polyurethane elastomers having mechanical properties which met the requirements of industrial practice.